Background <p>Periostitis ossificans, also known as Garré’s osteomyelitis, is a rare form of chronic osteomyelitis characterized by new bone formation with periosteal reaction. It most frequently affects the mandible, with common sources of infection being dental caries and periodontal lesions. This condition mainly affects younger patients. The therapeutic approach usually involves elimination of the infectious source, administration of antibiotics, and surgery if there is no response. This case is notable because no definitive source of infection was identified, which represents the main novelty of this report. The differential diagnosis with mandibular tumors or other inflammatory bone diseases made this case particularly challenging.</p> Case presentation <p>We report the case of a 13-year-old boy from Sfax, Tunisia, who presented with chronic osteomyelitis with proliferative periostitis of the mandible. He had no identifiable source of infection. Imaging and histological findings confirmed Garré’s osteomyelitis. The patient was successfully treated with intravenous antibiotics. At 10-month follow-up, he remained asymptomatic with a symmetrical facial morphology.</p> Conclusion <p>This case highlights the importance of considering Garré’s osteomyelitis in the differential diagnosis of mandibular swellings in adolescents, even in the absence of an evident infectious source. It also demonstrates the successful nonsurgical management of nonodontogenic Garré’s osteomyelitis.</p>